1. Field of the Invention
This invention relates to a method for the continuous catalytic dewaxing of lubricating oils.
2. Discussion of Related Art
In general, crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a number of subtractive unit operations which remove the unwanted components. The most important of these unit operations include distillation, solvent refining, and solvent dewaxing, which basically are physical separation processes in that all the separate fractions, if recombined, would reconstitute the initial crude oil.
Solvent dewaxing is a well-known effective process for removing the wax components, but it is expensive, and, solvent dewaxing merely removes wax, rather than converting the wax to lighter products. More recently catalytic methods for dewaxing have been proposed. U.S. Pat. No. Re. 28, 398, the entire disclosure is herein incorporated by reference, describes a catalytical dewaxing process wherein a particular crystalline zeolite is used.
Catalytic dewaxing processes not only remove the waxy components of hydrocarbon feedstocks, but also convert such components into other materials of higher value. Catalytic dewaxing processes achieve this end by selectively cracking long chain n-paraffins and some branched paraffins to produce low molecular weight products which may be removed by distillation.
Current technology for catalytically dewaxing petroleum feedstocks, having elevated pour points, involves the use of dewaxing reactors having trickle beds, whereby gas (primarily hydrogen) and the feedstock co-currently flow downward over a bed of solid catalyst. Typically, a single reactor is used. However, for low space velocity designs, multiple catalytic dewaxing reactors in series may be used to stay within allowable engineering designs, i.e. the lengths and weights used for a single reactor. The feedstock is to be catalytically dewaxed to a product having a targeted pour point of between about 20.degree. F. to -50.degree. F. (-6.7.degree. C. to -45.6.degree. C.). However, because of trace poisons in the feed such as nitrogen and metal, and by virtue of the occurrence of side reactions caused by the presence of olefins, the dewaxing catalyst gradually loses activity. This lost activity is usually compensated for by gradually raising the temperature of the dewaxing reactor. By raising the reactor temperature, a constant pour point product can be obtained as the catalyst loses activity. However, a temperature ceiling of about 675.degree. F. (357.degree. C.) is imposed on the process to maintain product quality and because the dewaxing catalyst rapidly deactivates above this ceiling. Thus, on reaching the ceiling temperature the reactor is shut down and the catalyst is reactivated.
The reactivation of dewaxing catalysts by hydrogen and oxygen reactivation is expensive because it causes down time which reduces the stream factor. That is, it reduces the time the reactor is on-stream. Stream factor is defined as the quotient of time of the reactor on-stream, divided by the total time the reactor is capable of use. Ideally this quotient is one. Unfortunately, in conventional dewaxing methods, the reactor is taken off-line for maintenance or for the reactivation of the catalyst. In such instances, the quotient is reduced to much less than one, thus increasing costs of making lube.
Hydrogen reactivation of the catalyst is accomplished by contact with elemental H.sub.2 at temperatures between 800.degree. F. and 1000.degree. F. (427.degree. C. to 538.degree. C.). Hydrogen reactivation usually removes soft deposits of coke. Hydrogen reactivation, however, does not completely restore the original level of activity of the catalyst. For example, it has been observed that following hydrogen reactivation of an HZSM-5 catalyst, the cycle length for catalytic dewaxing operations was substantially less than the original cycle length. The number of days the catalyst can remain on stream decreases from cycle to cycle and eventually continued reactivation becomes impractical. This loss of activity is trogen, sulfur and oxygen hetero atoms left on the catalyst which hydrogen activation does not remove. After a predetermined level of deactivation, oxygen is used to burn this hard coke residue off the catalyst and to restore most of the fresh catalyst activity. Catalyst reactivation is described in more detail in U.S. Pat. Nos. 3,904,510; 3,986,982; and 3,418,526.
It would be desirable to keep catalytic dewaxing reactors and the dewaxing cycle on-line or on-stream for longer periods of time to improve the stream factor and therefore the economics of catalytically dewaxing lubricating oils.
The present inventors have observed that in a two-stage catalytic dewaxing process, the dewaxing catalyst in stage-one or the first reactor shows a "line-out" aging in producing an intermediate pour point product, and the dewaxing catalyst in stage two or the second reactor shows a concave upward aging in dewaxing the intermediate pour product to a product having a targeted pour point. Thus, the stage-two dewaxing catalyst ages faster than the stage-one dewaxing catalyst and therefore the stage-two dewaxing catalyst is a limiting factor in the dewaxing cycle of the two-stage catalytic dewaxing processes. However, at the end of the cycle the stage-two dewaxing catalyst, even though it could no longer produce the target pour point, was found capable of catalytically dewaxing a feed to the intermediate pour point product while exhibiting the same line-out aging as the stage-one dewaxing catalyst. This aging data implies that in a conventional single stage catalytic dewaxing process, the aged catalyst from the back-end of the dewaxing reactor can be used at the front-end of the dewaxing reactor without reducing the performance of the catalytic dewaxing process. Replacement with a fresh or reactivated catalyst is only needed at the reactor back-end. This unique aging characteristic suggested to the inventors the feasibility of a continuous catalytic dewaxing process to bring the stream factor closer to one.